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| United States Patent |
5,232,795
|
|
Simon
, et al.
|
August 3, 1993
|
Rechargeable electrochemical cell having a solid electrolyte
Abstract
A rechargeable electrochemical cell comprising a cathode, an anode based on
a carbon-containing material capable of intercalating lithium ions, and an
electrolyte constituted by a cross-linked polymer incorporating a lithium
salt and a dipolar aprotic solvent. At least the surface of the
carbon-containing material is less crystallized than graphite so as to be
impermeable to said solvent, while allowing the lithium to diffuse.
| Inventors:
|
Simon; Bernard (Paris, FR);
Boeuve; Jean-Pierre (Marcoussis, FR)
|
| Assignee:
|
Alcatel Alsthom Compagnie Generale d'Electricite (Paris, FR)
|
| Appl. No.:
|
889234 |
| Filed:
|
May 28, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
429/312; 429/231.3 |
| Intern'l Class: |
H01M 010/40 |
| Field of Search: |
429/192,218,33,190
252/62.2
|
References Cited
U.S. Patent Documents
| 4303748 | Dec., 1981 | Armand et al. | 429/192.
|
| 4792504 | Dec., 1988 | Schwab et al. | 429/192.
|
| 4830939 | May., 1989 | Lee et al. | 429/192.
|
| 5028500 | Jul., 1991 | Fong et al. | 429/218.
|
| 5069683 | Dec., 1991 | Fong et al. | 429/218.
|
| Foreign Patent Documents |
| 9013924 | Nov., 1990 | CA.
| |
| 0404578 | Dec., 1990 | EP.
| |
Primary Examiner: Skapars; Anthony
Attorney, Agent or Firm: Sughrue, Mion, Zinn Macpeak & Seas
Claims
We claim:
1. In a rechargeable electrochemical cell comprising a cathode, an anode
based on a carbon-containing material capable of intercalating lithium
ions, and an electrolyte constituted by a cross-linked polymer
incorporating a lithium salt and a dipolar aprotic solvent, the
improvement wherein said anode is constituted by a mixture of said
electrolyte and said carbon-containing material, with at least the surface
of the carbon-containing material being less crystallized than graphite so
as to be impermeable to said solvent, while allowing the lithium to
diffuse therein.
2. A electrochemical cell according to claim 1, wherein said
carbon-containing material is selected from the group consisting of cokes,
graphitized carbon fibers, and pyrolytic carbons.
3. An electrochemical cell according to claim 1, wherein the surface of
said carbon-containing material includes a CVD hydrocarbon deposit.
4. An electrochemical cell according to claim 1, wherein the surface of
said carbon-containing material includes a carbonized polymer film deposit
such as polyacrylonitrile.
5. An electrochemical cell according to claim 1, wherein said electrolyte
is a cross-linked conductive polymer incorporating a metal salt having an
anion selected from the group consisting of AsF.sub.6.sup.-,
BF.sub.4.sup.-, PF.sub.6.sup.-, CF.sub.3 SO.sub.3.sup.-, ClO.sub.4.sup.-,
B(C.sub.6 H.sub.5).sub.4.sup.-, N(CF.sub.3 SO.sub.2).sub.2.sup.-, and
SCN.sup.-, and a non-aqueous solvent selected from the group consisting of
ethylene carbonate, propylene carbonate, tetrahydrofuran,
dimethylcarbonate, diethylcarbonate, methyl formate, dimethylsulfite,
deimethylsulfoxide, 1-2 dimethoxyethane, sulfolane, g-butyrolactone,
polyethylene glycol dimethylether, and mixtures thereof.
6. An electrochemical cell according to claim 5, wherein said cross-linked
conductive polymer is a polyether selected from the group consisting of
polyethylene oxide and polypropylene oxide, and copolymers thereof.
7. An electrochemical cell according to claim 1, wherein said cathode is a
composite electrode constituted by a material having a high redox
potential, said electrolyte and a conductive powder selected from the
group consisting of carbon black and graphite.
8. An electrochemical cell according to claim 7, wherein said material
having a high redox potential is one material selected from the group
consisting of LiV.sub.2 O.sub.5, LiCoO.sub.2, polyaniline and polypyrrol,
in the reduced state and doped with lithium ions.
Description
The present invention relates to a rechargeable electrochemical cell having
a solid electrolyte, and having a negative active material based on
lithium.
Developing such an "all-solid" high potential rechargeable cell is of
considerable interest given the major advantages it can offer:
flexibility, no risk of leakage, and no separator.
To make the electrolyte for such a cell, it is possible to use
ion-conducting polymers which can only be used at temperatures higher than
ambient temperature. For example, U.S. Pat. No. 4,303,748 describes
plasticized polyethylene oxide (PEO) incorporating a metal salt, but
requiring an operating temperature of 80.degree. C. U.S. Pat. No.
4,830,939 describes a cell containing an electrolyte of that type.
In order to lower the operating temperature to ambient temperature, U.S.
Pat. No. 4,792,504 describes using cross-linked polymers containing an
ion-conducting phase comprising a metal salt and a dipolar aprotic
solvent. U.S. Pat. No. 2,653,938 describes a lithium anode cell using such
a polymer. However, during cycling, the lithium electrode undergoes
changes in its shape due to the growth of dendrites which have detrimental
effects such as the risk of short-circuits, and diminished cycling
efficiency.
In order to avoid those drawbacks, consideration has been given to
replacing the metallic lithium anode by an anode made of graphite with the
lithium being intercalated therein (see article R. Yazami, Ph. Touzain, J.
Power, Sources 9, 1983, p. 365). However, it has been observed that
associating natural or artificial graphite with the above-mentioned
ion-conducting polymers containing solvents leads to a drop in capacity
during operation, and to considerable swelling of the anode, and this may
give rise to exfoliation of the graphite.
An aim of the present invention is to provide a rechargeable
electrochemical cell of the preceding type, but in which the carbon-based
anode retains its qualities during cycling.
The present invention provides a rechargeable electrochemical cell
comprising a cathode, an anode based on a carbon-containing material
capable of intercalating lithium ions, and an electrolyte constituted by a
cross-linked polymer which is preferably a conductor of ions and which
incorporates a lithium salt and a dipolar aprotic solvent, said cell being
characterized by the fact that said anode is constituted by a mixture of
said electrolyte and said carbon-containing material, with at least the
surface of the carbon-containing material being less crystallized than
graphite so as to be impermeable to said solvent, while allowing the
lithium to diffuse.
In a first embodiment, said carbon-containing material is chosen from
cokes, graphitized carbon fibers, and pyrolytic carbons. Such carbons have
small crystallites, and a certain number of structural faults, such as
interstitial carbon atoms between the planes, or mutually disorientated
planes.
The anode of the cell of the invention prevents solvent co-intercalation
which is responsible for swelling during cycling. However, it should be
noted that structural imperfections limit the intercalation kinetics of
lithium, and therefore it may be advantageous to use a material based on
highly crystallized graphite having its surface covered with a thin layer
of incompletely graphitized carbon.
Such a layer may be obtained by carbonizing a film of polymer, such as
polyacrylonitrile, or by chemical vapor deposition (CVD) using a
hydrocarbon, e.g. methane or benzene.
The electrolyte polymer is preferably a cross-linked ion-conducting polymer
of the type described in U.S. Pat. No. 2,653,938.
Such a polymer incorporates a conductive salt whose cation is lithium and
whose anion is preferably chosen from AsF.sub.6.sup.-, BF.sub.4.sup.-,
PF.sub.6.sup.-, CF.sub.3 SO.sub.3.sup.-, ClO.sub.4.sup.-, B(C.sub.6
H.sub.5).sub.4.sup.-, N(CF.sub.3 SO.sub.2).sub.2.sup.-, and SCN.sup.-.
The non-aqueous solvent is preferably chosen from ethylene carbonate,
propylene carbonate, tetrahydrofuran, dimethylcarbonate, diethylcarbonate,
methyl formate, dimethylsulfite, deimethylsulfoxide, 1-2 dimethoxyethane,
sulfolane, g-butyrolactone, polyethylene glycol dimethylether, and
mixtures thereof.
The cathode is a composite electrode constituted by a material having a
high redox potential (e.g. such as LiV.sub.2 O.sub.5, LiCoO.sub.2,
polyaniline and polypyrrol, in the reduced state and doped with lithium
ions), said electrolyte, and optionally a conductive powder (e.g. such as
carbon black or graphite).
Other characteristics and advantages of the present invention will appear
on reading the following description of an embodiment given by way of
non-limiting example.
An electrolyte, an anode, and a cathode were prepared as follows:
The solid polymer electrolyte was made by mixing 0.5 g of polyoxyethylene
(20,000)bis(amine), 0.1 g of LiAsF.sub.6, 15 .mu.l of ethylene glycol
diglycidyl ether as a setting agent, and 0.7 g of propylene carbonate as a
plasticizer.
Cross-linking took place at 70.degree. C. for 16 hours.
The anode was obtained in the same way, with 2 g of the carbon powder sold
under the name M-2007S (Kureha) being added to the mixture.
The cathode was obtained in the same way, with 1.5 g of LiCoO.sub.2 (sold
by Aldrich) and 0.03 g of carbon black being added to the mixture. Prior
to cross-linking, the mixture was deposited on a titanium collector to
form a layer of desired width (about 1 mm).
A button cell was made by stacking the three preceding components
(anode-electrolyte-cathode) in the form of wafers having a diameter of 15
mm, and thicknesses of 1 mm for the anode, 0.05 mm for the electrolyte and
1 mm for the cathode.
The cell was charged to 4.2 V at a current density of 0.25 mA/cm.sup.2.
The cycled capacity between 3 V and 4.2 V was 150 mAh/g of carbon in the
anode, and remained stable for more than 500 cycles.
Naturally, the invention is not limited to the above-described embodiment.
Any means may be replaced by equivalent means without going beyond the
ambit of the invention.
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